In the art of producing droplets of controlled size and distribution, there are two basic approaches, which may be described as "drop on demand" and "vibration of a continuous pressurized flow".
Drop on demand produces a single droplet or fewer from an orifice for each impulse of an impulse applicator, e.g. of a piezoelectric crystal such as that of Ashgrizzadeh and Yao, "Development of Multi-orifice Impulsed Spray Generators for Heterogeneous Combustion Experiments", Carnegie-Mellon University, Pittsburgh, Pa., Proc. ASME/JSME Thermal Engineering Joint Conf., Honolulu, Mar. 20-24, 1983, p. 433-439 (hereinafter referred to as Yao). This technique is used in ink jet technology, and has been proposed for production of metal droplets in "Preparation of Monosized Metal Powders by Pulsated Orifice Injection", by Akira Kawasaki, Yasunori Kuroki and Ryuzo Watanabe, Proceedings of 1993 Powder Metallurgy World Congress, .COPYRGT.Japan Society of Powder and Powder Metallurgy, p. 27-30.
Vibration of a continuous flow, see U.S. Pat. No. 4,428,894, and also described in Yao, is presently under consideration for industrial production of metal droplets. A continuous flow from an orifice is produced by a pressure applied to a fluid in a container and as the flow exits an orifice it is vibrated to cause instabilities in the fluid and therefore break it up into droplets. The pressure on the fluid produces a jet, which breaks up by Rayleigh instability into droplets. The length of the jet before it breaks up may be determined from the work of Harmon, Darell B., "Drop Sizes from Low Speed Jets", Journal of the Franklin Institute, 259 (1955), pp. 519-522. The jet is itself continuous with the fluid in the container. The droplets produced from the disintegrated jet are nearly spherical.
One of the main problems with the method of vibrating a continuous jet of molten metal or alloy is that the formed droplets tend to stick to one another in flight. This likely occurs because of the close proximity, in direction of flight, of the drops formed from a continuous jet vibrated at high frequency. Although the mechanism is not fully understood, it may be that since the droplet separation distance is typically quite small in such jets, the formation of smaller satellite drops will lead to coalescence which must be prevented or reduced by some particular means provided for that purpose. Thus, in one instance in the prior art, the drops are passed through a charging plate, once they are formed, so that they may be electrically charged and remain separate from one another. The use of such additional equipment makes the process for producing droplets more complex, costly and in some cases dangerous for adaption to a large-scale industrial plant. Also, by using overpressure to create continuous streams of metal it becomes more difficult to apply these techniques to discrete parts manufacturing since it is more difficult to start and stop the process.
For drop on demand, the droplets produced are single droplets believed to have a length that is less than .pi. times their diameter, as seen in the work of Kawasaki, or as may be calculated from volume conservation considerations from Yao. The technique as practised by Yao has not proven itself to produce monodisperse droplets, and does not permit flexibility in the control of the shape of the produced droplets. In addition, the stroke length of the impulse applicators of Yao and Kawasaki are very low, since they are produced by piezoelectric crystals, in the order of about 0.065 .mu.m.